1. Field of the Art
The present invention relates to a light modulation device in which light is made to pass through two parallel optical waveguides made of a compound semiconductor to apply modulation voltages to the optical waveguides, thereby varying the intensity of the light.
2. Prior Art
When light is made to pass through optical waveguides disposed sufficiently adjacent and in parallel, light energy is exchanged between the optical waveguides due to the coupling between the optical waveguides. That is, as the light travels in the optical waveguides, a portion or all of the light energy in one of the waveguides transfers to the other of the optical waveguides. As the light travels farther, the light energy returns to the waveguide in which it was before. In this manner, the light energy moves reciprocally between the optical waveguides as the light travels therein.
In the case where the optical waveguides are made of a material having an electrooptical effect, when voltages are applied to the optical waveguides, refractive indices of the waveguides vary corresponding to the voltages applied. Then, since the coupling condition changes, the exchange of the light energy between the optical waveguides can be controlled by the applied voltages. The output light of a single optical waveguide varies corresponding to variation in the voltage applied thereto. Accordingly, a light modulation device can be constituted by applying voltages to optical waveguides coupled in parallel.
An example of the light modulation device of this kind is shown in FIG. 4.
A light modulation device generally designated 30 in FIG. 4 comprises a LiNbO.sub.3 substrate 31 with two optical waveguides 32 and 33 having high refractive indices formed thereon by selective diffusion of titanium Ti. Electrodes 34 and 35 are formed on the optical waveguides 32 and 33, respectively. Modulation voltages 36 and 37 are applied to the electrodes 34 and 35, respectively, to reciprocally vary the refractive indices of the optical waveguides, respectively.
The two optical waveguides 32 and 33 are sufficiently adjacent to each other to allow exchange of light energy therebetween. The coupling condition between the optical waveguides varies corresponding to the modulation voltages to vary the intensity of the light passing therethrough.
However, the light modulation device 30 has the following disadvantages:
(a) Since the voltages are applied not only to the optical waveguides 32 and 33, it is necessary to apply sufficiently large voltages. Since there is no difference in resistivity between the LiNbO.sub.3 substrate and the optical waveguides 32, 33 with Ti selectively diffused therein, the greatest portion of the voltages is applied to the LiNbO.sub.3 substrate. The voltage required for modulation is preferably smaller.
(b) Since the electrodes 34, 35 are in contact with the optical waveguide 32, 33, loss of light by absorption is large there.
(c) Since the substrate is LiNbO.sub.3 instead of a compound semiconductor, it is impossible to mount this light modulation device as well as other optical circuit components such as a light source, a light detector and the like on the same substrate. In other words, it cannot be used as a component of a monolithic optical integrated circuit.
The light modulation device described above is disclosed by, for example, an article by R. V. Schmidt and H. Kogelnik entitled "Electro-optically switched coupler with stepped .DELTA..beta. reversal using Ti-diffused LiNbO.sub.3 waveguides" in the Applied Physics Letters, Vol. 28, No. 9, 1 May 1976, pp. 503-506.
There has been proposed a light modulation device directed to be monolithic by using a compound semiconductor such as GaAs substrate. An example of publicly known light modulation device using GaAs substrate is shown in FIG. 5.
A light modulation device generally designated 40 in FIG. 5 comprises an n-type GaAs substrate 41 having an i-type GaAs layer 42 formed thereon and two parallel bar members 43, 44 of p-type GaAs formed on said i-type GaAs layer 42. Electrodes 45, 46 and 47 are provided on the substrate 41, and the bar members 43, 44, respectively.
The i-type GaAs layer (intrinsic semi-conductor) 42 having high resistance can capture light and function as an optical waveguide layer.
Further, since the p-type GaAs bar members 43, 44 are disposed on the i-type GaAs layer 42, regions 49, 50 directly under the bar members 43, 44, respectively, have equivalently high refractive indices. Since light is captured in the regions 49, 50, they function as optical waveguides.
The electrode 45 is grounded. A negative D.C. voltage is applied to the electrodes 46, 47, and further a modulation voltage is applied thereto. Since the p-n junction is reversally biased, no current is allowed to flow therebetween.
Since the optical waveguides 49, 50 are made of i-type GaAs having a low carrier density, the greater portion of the voltage is applied to the optical waveguides and little voltage is applied to the substrate 41 and the bar members 43, 44.
The light modulation device shown in FIG. 5 has advantages that it can be used as a component of a monolithic optical integrated circuit since it uses GaAs as substrate, and that since the regions 49, 50 functioning as optical waveguides are spaced from the electrodes 46, 47, there is very little loss of light by absorption by the electrode metal and the greater portion of the applied voltage is applied to the optical waveguide regions 49, 50.
On the other hand, however, the light modulation device shown in FIG. 5 has disadvantages described below.
The light modulation device 40 is produced by epitaxial growth of an i-type GaAs layer 42 and a p-type GaAs layer on an n-type GaAs wafer. Thereafter, the p-type GaAs layer is etched to leave two parallel square bar-shaped portions which should function as the bar members 43, 44.
Ideally, etching should be performed accurately to the boundary between the p-type and the i-type layers. In fact, however, since the p-type and the i-type are both of the same GaAs crystal and different only in impurity density, there is no substantial difference in etching rate therebetween. Accordingly, the p-type layer is etched, with the etching time constant, to the boundary between the p-type and the i-type layers to remove the portions other than the bar members 43, 44.
In case of over-etching, the i-layer is etched to remove the portion between the two optical waveguides 49, 50, thereby preventing generation of coupling of the waveguides. In case of under-etching so as not to remove the i-layer, a portion of the p-layer is left unetched on the i-layer. FIG. 6 is a sectional view of the light modulation device showing the state of under-etching described above.
In the under-etched light modulation device shown in FIG. 6, residual p-layers 51, 52, 53 are present adjacent to the p-type bar members 43, 44. Since a p-layer is of a lower specific resistance than an i-layer, the residual p-layers 51, 52, 53 are substantially equipotential to the bar members 43, 44. The voltage is applied widely between the p-layers and the n-type substrate 41, whereby it is no more possible to concentrate the applied voltage to the optical waveguide regions 49, 50 only.
The voltage of the D.C. component to inversely bias the p-n junction is supplied widely between the p-layers and the n-type substrate, and is applied to all over the i-layer.
The modulation voltages are, unlike the voltage of the D.C. component, applied to the electrodes of the p-type bar members in opposite polarities. These A.C. components are, however, short-circuited by the intermediate residual p-layer 52 having a low resistance. Accordingly, the modulation electric field hardly extends into the optical waveguide regions 49, 50 thereby prohibiting modulation.
As described above, the light modulation device shown in FIG. 5 is difficult to manufacture because its etching is very delicate in process.
The light modulation device described above is disclosed by, for example, an article by A. Carenco and L. Manigaux entitled "GaAs homojunction rib waveguide directional coupler switch" in the J. Appl. Phys, 51(3), March 1980, pp. 1325-1327.
The following properties are generally required for a light modulation device:
(a) Light can be captured in the optical waveguide layer; and
(b) Most of the applied voltage is applied locally to the optical waveguide layer.
In the conventional light modulation device shown in FIG. 5, in order to capture the light, difference in carrier density is produced in the GaAs crystal to utilize the phenomenon that refractive index is higher in the portion lower in carrier density. For this reason, it has a p-i-n structure. That is, the condition (a) is accomplished by utilizing the difference in carrier density. The condition (b) is also related directly to specific resistance or carrier density of each of the layers. The i-layer (intrinsic semiconductor) has a higher specific resistance than any of the n- and the p-layers. In using the i-layer as the optical waveguide, this is not contradictory to the condition (b) because in the laminated structure of the p-i-n layers, when voltages are applied to the p-n junction in opposite directions, most of the voltage is applied to the i-layer.
In this known technique, the difference in carrier density is produced skillfully in the GaAs crystal to satisfy the conditions (a) and (b) at the same time.
However, since only one parameter, that is carrier density, is used to satisfy the two conditions, this known light modulation device has a disadvantage that the number of parameters is insufficient to provide satisfactory control.
The difficulty in the etching process described hereinabove is, after all, due to the insufficient number of parameters.
The inventors have found that the above-mentioned disadvantage can be overcome if the two conditions required for the optical waveguides of a light modulation device are satisfied independently from each other by operating separate parameters.